This invention relates generally to a process for fabricating semiconductor devices, and more specifically, to a process for patterning organic layers on semiconductor devices.
Polyimide and other organic materials are finding increased usage in semiconductor device processing as dielectric materials and especially in applications where the dielectric material provides a planarization of the semiconductor wafer. One particular process in which polyimide material has found application is in the process for fabricating devices having multiple layers of interconnection. For example, in a complex integrated circuit the interconnection of all devices and all device functions may require more than one layer of metallization. In processing the device, a first layer of metallization, such as aluminum, is formed overlying the surface of the semiconductor device and interconnecting selected ones of device areas on the structure. A layer of organic insulator is then applied over the first layer metallization and openings are formed through the insulator at selected locations to allow interconnection between the first and subsequent interconnecting layes. A second layer of interconnecting material is then deposited and patterned on the relatively planar surface of the organic insulator to contact and interconnect further device functions. The two layers of interconnection are electrically isolated from each other by the intervening insulator layer except at locations where intentional contact between the layers is effected through one of the openings. Subsequent layers of interconnection, as required, are similarly selectively separated by layers of organic insulation. A layer of organic insulator may also be used as a final passivating layer over the otherwise completed device.
As circuit functions become more complex, device sizes decrease and device density increases. This requires smaller and smaller feature sizes including the width and spacing of interconnecting lines. In order to process devices having these fine interconnecting lines with a high degree of reproducability and reliability, it is necessary that the surface upon which the lines are formed be relatively planar. In addition, to insure the desired isolation between interconnection layers, the insulating material must be relatively thick. The combination of the two requirements dictates that the layer itself provide some degree of surface leveling. Materials such as polyimides are particularly suited for such an application. The material is applied in thick layers as a liquid and, regardless of the topography of the underlying layer, can provide a relatively flat surface upon which subsequent interconnection layers can be formed.
The requirement of small feature sizes with close spacing between adjacent features further requires that an anisotropic etch technique such as reactive ion etching be used to form the openings through the thick insulating layer positioned between adjacent layers of interconnecting material. Reactive ion etching, however, although providing the desired degree of anisotropy, presents problems with the device processing. One particular problem, for example, relates to the sputter etching and subsequent redeposition of materials during the reactive ion etch processing. This problem is encountered especially in those applications in which the thickness of the organic layer varies considerably over the surface of the device, as is common where the layer is used for planarization. Some underlying portions of the device which have a relatively thin layer of organic material thereover may be exposed to the reactive ion etching while openings are still being formed through other relatively thicker portions of the organic layer. That is, some portions of the pattern of openings through the organic layer will be subjected to a considerable overetch in order to insure that openings are properly formed through the thicker portions. During this overetch time, the material underlying the opening is subjected to the reactive ion etching. If the underlying material is aluminum metallization, for example, and the organic layer is being reactive ion etched in oxygen, aluminum may be sputter etched from the surface of the exposed aluminum metallization. The sputtered aluminum, in turn, can chemically react with the oxygen to form aluminum oxide which is redeposited on the walls of the opening formed through the organic material. The aluminum oxide is a relatively inert and chemically resistant material which is relatively unaffected by subsequent processing of the organic material. The result is a "picket fence" lining each of the openings through the organic material which can seriously interfere with the reliability of the device. The aluminum oxide picket fence is relatively unaffected by subsequent etching of the layers and is therefore left standing around the opening when the organic material is etched. The picket fence may also partially collapse into the opening during subsequent processing, causing at least a partial blockage of the opening. It has been suggested that the redeposited material be subsequently removed by etching in a liquid etchant which is selected dependent upon the chemical nature ot the redeposited material. For example, if the underlying material which has been sputter etched is aluminum, the redeposited material is etched with a mixture of ethylene glycol and buffered hydrofluoric acid. If the underlying material is gold, the redeposited material is removed in an aqueous solution of potassium iodide and iodine. The use of such liquid etchants, however, is not an acceptable solution, because the liquid etchant attacks and seriously degrades the underlying substrate material. In the case of aluminum metallization, for example, the aluminum oxide is removed, but only at the expense of also etching the underlying exposed aluminum metallization.
In view of the foregoing problems, a need existed for an improved process for patterning organic layers using anisotropic reactive ion etching.
It is therefore an object of this invention to provide such an improved process for the reactive ion etching of organic layers.
It is another object of this invention to provide an improved process for the reactive ion etching of polyimide layers.
It is a still further object of this invention to provide an improved process for the fabrication of semiconductor devices.